The invention relates to an improved process for the heat treatment of liquid milk products including food products based on milk and containing starch or other carbohydrates. It is particularly concerned with a process for the treatment of fresh cream, fresh milk, skim milk, flavored milk, lactose reduced milk and other liquid milk products to prolong the shelf-life of such products.
It is well-known that the storage life of cream, milk and other liquid milk products is often very short when these products are stored at ambient temperature, e.g. 10° to 30° C., and that the shelf-life of such products can be prolonged by storing the products under refrigerated conditions, e.g. 5° to 10° C. Such refrigeration prolongs the shelf-life of the liquid milk products by up to 10-20 days.
The deterioration in the quality of liquid milk products such as cream and milk is due to microbiological activity that normally develops within a few days following storage to a level such that the product takes on unacceptable flavor characteristics and frequently undergoes unacceptable physical changes. The microbiological activity that gives rise to these unacceptable changes is not prevented by conventional pasteurization treatment and it has been proposed to subject dairy products to higher temperature heat treatments in order to inhibit this microbiological activity. Such heat treatment may involve flash-heating to about 145° C. (293° F.), the so-called ultra high temperature (UHT) treatment. Milk can be made commercially sterile by subjecting it to temperatures in excess of 100° C., and packaging it in air-tight containers. The milk may be packaged either before or after sterilization. The basis of UHT, or ultra-high temperature, is the sterilization of food before packaging, then filling into pre-sterilized containers in a sterile atmosphere. Products which have been heat-treated in this way have prolonged shelf-lives of several months. However, products treated in this manner suffer from the severe disadvantage that they lose their natural, fresh taste and take on a characteristic burnt taste that is less attractive to the consumer.
It has also been proposed to improve the keeping properties of fresh milk by subjecting the milk to a short-time heat treatment at 100° C.-145° C. (212° F.-293° F.), preferably about 105° C. (221° F.), followed by packaging of the heat-treated milk at a temperature of 70° C. to 80° C. (158° F.-176° F.) and then subsequently cooling the packaged milk in a predetermined manner. However, from a practical stand point, by operating at the preferred temperature of 105° C. (221° F.), no significant prolongation of shelf-life is obtained, while if the product is given a conventional UHT treatment it immediately takes on the unacceptable “sterilized” or “burnt” flavor.
Shelf stable and aseptic concentrate milks available in the market place today use a condensing technology to reduce the water content of the starting milk material that involves the use of a pre-heat treatment or pasteurization. They are traditionally sold as “condensed milks” packaged in cans or packages. These products are condensed by a heating process and as a result, there is substantial damage to the structure of the milk, which affects the ability of the concentrated product to be rehydrated. Furthermore, condensation by heating results in a lowering of pH (increase in acidity) that contributes to the faster spoilage of the concentrated product, and damages the structural integrity of the product. Additionally, the increased acidity of heat-based concentrated products results in denaturation of the milk components, for e.g., milk proteins, which in turn causes the milk product to separate out into a particulate phase and a liquid phase. Such phase-separated products are visually unappealing and must be thoroughly shaken prior to use or consumption.
Additionally, the flavor profile of the heat-concentrated product is different from that of the raw milk or skim milk starting material. Since these products typically have additional heat applied to them by the UHT pasteurization process, they acquire a “burnt” or “sterilized” flavor that is disfavored by consumers.
Consequently, there is a need for a heat-treatment process that is capable of inhibiting the microbiological activity in liquid milk products to an extent which will permit the product to be stored without refrigeration, for prolonged periods of time, e.g. in excess of four (4) weeks and up to six (6) months. There is also a need for processes that produce liquid milk products that have extended shelf lives without suffering from the ills of microbial spoilage. Furthermore, there is a need for a process that produces an aseptic milk product using heat treatment steps, while at the same time avoiding the difficulties of the unacceptable “burnt” or “sterilized” flavor.
The present invention is based upon the premise that by using non-heating processes for the concentration of the raw material coupled with the careful selection of the heat-treatment temperature and time of exposure, it is possible to inhibit microbiological activity in the treated product when it is stored in sealed containers at 10° C. to 30° C. (50° F.-86° F.) for long periods of time while, at the same time, producing a product that is free from the characteristic “burnt” flavor and whose taste is indistinguishable from the fresh product.